As integrated circuits (ICs), such as digital signal processors (DSPs) and mixed signal analog circuits, are required to operate at faster speeds, and continue to be designed and built smaller, more electrical power is used per unit of wafer surface area. Using more electrical power per unit of wafer surface area causes the operating temperature of ICs to increase. Since the power consumed by a device is a function of the current flowing through the IC and the upon the voltage placed across the IC, lower working voltages are sometimes implemented to lower the power used by the IC.
Likewise, as overall system sizes have continued to decrease, power supply designs with smaller sizes and higher efficiencies are used to supply power to these smaller circuits that operate at lower voltages. Switch mode power supplies (also known as direct current (DC) to DC converters, are typically used in electrical devices, such as digital signal processors (DSPs) and mixed signal analog circuits. These devices are commonly used in products such as modems, television receivers and cell phones, for example. DC to DC converters convert the voltage of a primary source (such as a device battery) to the operating voltage required by the electrical circuitry or IC of the electrical device. Likewise, it is often necessary to re-convert the voltage of the electrical device from the voltage of the electrical circuitry or IC to a higher operating voltage used by the device. For instance, a 2.8 volt supply voltage provided to a BiCMOS IC may need to be increased internally to 5.0 volts to operate internal CMOS circuitry. Sometimes, a charge-pump is used to generate a larger power supply voltage from a smaller one.
Unfortunately, the performance of a charge-pump is not very efficient during the charge-up period of the charge-pump. In addition, during the charge-up period, an IC coupled to the charge-pump may be damaged due to an over-voltage condition.
Also, the performance of charge-pumps is limited due to the time it takes to charge-up the charge-pump. Furthermore, if the output voltage of the charge-pump exceeds it's input voltage, the charge-pump may forward bias back diodes and inject noise into the IC. All of these factors contribute to reducing the efficiency of the charge-pump.
Therefore it is desired to have a charge-pump circuit and a method for implementing a charge-pump that increases start-up speed, increases charge-pump efficiency, prevents over-voltage conditions, and reduces the likelihood of noise injection. The present invention provides such a device and method.